The present invention relates to thermoplastic preforms, of the type used for blow molding polyester beverage bottles, and more particularly to preforms having a crystallized first portion (e.g., neck finish) for resistance to deformation at elevated temperatures.
There are a variety of applications in which thermoplastic containers are subjected to elevated temperatures. These include hot-fill containers, which must withstand filling with a hot liquid product (for sterilization purposes) without deformation, followed by sealing and a cooling process which produces a vacuum (negative pressure) in the container. Another application is pasteurizationxe2x80x94a pasteurizable container is filled-and sealed at room temperature, and then exposed to an elevated temperature bath for about ten minutes or longer. The pasteurization process initially imposes high temperatures and positive internal pressures, followed by a cooling process which creates a vacuum in the container. Throughout these procedures, the sealed container must resist deformation so as to remain acceptable in appearance, within a designated volume tolerance, and without leakage. In particular, the threaded neck finish must resist deformation which would prevent a complete seal.
Another high-temperature application is use as a returnable and refillable carbonated beverage container, now commercialized in Europe, South America, and Asia. In this application the container must withstand twenty or more wash and reuse cycles in which it is filled with a carbonated beverage at an elevated pressure, sold to the consumer, returned empty, and washed in a hot caustic solution prior to refilling. These repeated cycles of exposure to hot caustic agents and filling at elevated pressures make it difficult to maintain the threaded neck finish within tolerances required to ensure a good seal.
A number of methods have been proposed for strengthening the neck finish portion of a container to resist deformation at elevated temperatures. One method is to add an additional manufacturing step whereby the neck finish of the preform or container is exposed to a heating element in order to thermally crystallize the neck finish. However, there are several problems with this approach. First, during crystallization the polymer density increases, which produces a volume decrease. Therefore, in order to obtain a desired neck finish dimension, the as-molded dimension must be larger than the final crystallized dimension. It is difficult to achieve close dimensional tolerances with this method. In general, the variability of the critical neck finish dimensions after crystallization are approximately twice that prior to crystallization. Secondly, there is the increased cost of the additional processing step which requires both time and the application of energy (heat). The overall cost of producing a container is very important and tightly controlled because of competitive pressures.
Alternative methods of strengthening the neck finish involve crystallizing select portions of the neck finish, such as the top sealing surface and flange. Again, this requires an additional heating step. Another alternative is to use a high Tg material in one or more layers of the neck finish. Generally, this involves more complex preform injection molding procedures to achieve the necessary layered structure in the finish.
Thus, it would be desirable to provide a thermoplastic preform for a container having a neck finish which resists deformation, particularly at elevated temperatures, and a commercially acceptable method of manufacturing the same.
The present invention is directed to a method for making a preform with a crystallized first portion (e.g., neck finish) and an amorphous second portion (e.g., body-forming portion). The method is both cost effective and can provide a thermal-resistant neck finish within a given dimensional tolerance.
In one embodiment, a method of making the preform is provided wherein a first thermoplastic material is introduced into a first mold section to form a crystallized neck portion of a preform, and a second thermoplastic material having a relatively low crystallization rate compared to the first material is introduced into a second mold section to form a substantially amorphous body-forming portion of the preform. By achieving crystallization in the neck finish during the molding step, the initial and final finish dimensions are the same so that dimensional variations are minimized. Also, a higher average level of crystallization in the finish can be achieved by utilizing the higher melt temperatures and/or elevated pressures of the molding process. Furthermore, by crystallizing during the molding step, the prior art step of post-mold thermal crystallization can be eliminated.
Another aspect of the invention provides a method and apparatus for the cost-effective manufacture of such preforms. In one embodiment, the apparatus includes an indexer (e.g., rotary or oscilliatory) with two faces, each face having a set of preform molding cores. The cores on the two faces are simultaneously positionable in two different sets of preform molding cavities. In a first set of cavities (first molding station), a crystallized neck portion is being formed on one set of cores, while in the other set of cavities (second molding station) a plurality of amorphous body-forming portions are being formed on the other set of cores. By simultaneously molding in two sets of cavities, an efficient process is provided. By molding the neck and body-forming portions separately in different cavities, different temperatures and/or pressures may be used to obtain different molding conditions and different properties in the two preform portions. For example, it is possible to render the neck portion opaque by thermally crystallizing the neck portion in the first set of cavities, while maintaining the body-forming portion substantially amorphous in the second set of cavities.
Various thermoplastic polymers can be used to form the neck and body-forming portions, and the processing conditions will vary depending on the specific application. In one embodiment, a hot-fillable polyester container is made having a crystallized neck portion of CPET, a terephthalic polyester with nucleating agents which render the polymer rapidly crystallizable during injection molding. The body-forming portion is a two-material, three-layer (2M, 3L) structure, including inner and outer layers of virgin polyethylene terephthalate (PET), and a core layer of for example post-consumer PET (PC-PET). Numerous alternative high glass transition (Tg) polymers may be used in place of CPET, such as arylate polymers, polyethylene naphthalate (PEN) homopolymers, copolymers or blends, polycarbonates, etc. As for the body-forming portion, numerous alternative polymers and layer structures are possible, incorporating PEN, ethylene/vinyl alcohol (EVOH) or MXD-6 nylon barrier layers, oxygen scavenging polymers, etc.
The present invention will be more particularly set forth in the following detailed description and accompanying drawings.